1. Field of the Invention
This invention relates to polypropylene fibers having an improved heat-shrinkability and tenacity. More particularly it relates to polypropylene fibers produced from a specified polypropylene resin and having an improved tenacity and an improved shrinkability in a heated atmosphere.
2. Description of the Prior Art
In general, fibers consisting of polypropylene resin have been first prepared by melt-extruding the resin through various shapes of die, then processed into filaments, staple fibers, flat yarns, etc. via stretching step, heat-treatment step, etc., and further secondarily processed into waddings, carpet piles, non-woven fabrics, industrial materials, striped fabrics, cloth-like products, etc.
Fibers consisting of polypropylene resin have suitable tenacity characteristics imparted by orientation-crystallization during their spinning and stretching steps and have been used for practical uses, but they have such drawbacks that their tenacity is liable to be reduced and their shrinkage is liable to occur.
Thus in order to prevent shrinkage of their products with increase of time when they are preserved and used at room temperature, the fibers have been usually subjected to relaxation heat treatment at a temperature lower than the melting point of polypropylene after their stretching, to remove their internal strain formed when they are oriented during the spinning and stretching steps, i.e. their residual stress, which is a cause of the shrinkage and promote recrystallization to thereby stabilize the shrinkage. On the other hand, in order to make up the fibers into products, various secondary processings are required, and in the case of some products, the fibers are often subjected to various processes exposed to an atmosphere at higher temperatures than room temperature; in particular, at higher temperatures than the heat treatment temperature, there occurs retrogradation of the orientation at the time of spinning and stretching whereby the shrinkage is rapidly increased.
Referring to flat yarns used as a primary backing of carpets, for example in the case of tufted carpets, polypropylene fibers are tufted on the backing and backed with a latex, followed by a latex-drying step; hence the fibers are exposed to a heated atmosphere at considerably high temperatures. Further, recently there is a tendency that the latex-drying step is carried out at higher temperatures and higher speeds to improve the productivity of the products. For example, when the fibers are allowed to stand at 130.degree. C. for 15 minutes, if they have a heat-shrinkability endurable to the conditions, no problem has so far been raised, but a heat-shrinkability endurable to higher temperatures than such a temperature has recently come to be required.
In order to obtain a low heat-shrinkability under heating, i.e. the so-called low shrinkage, relaxation annealing may be generally applied after stretching, as described above, but the percentage relaxation has so far been generally 10 to 25%; if the percentage exceeds such values, a problem is raised that the productivity is reduced as much as the increase in the percentage relaxation.
Further, the percentage heat shrinkage of flat yarns is said to depend on the shrinkage of their noncrystallized portion caused by crystallization under heating, the recovery of the internal strain formed at the time of orientation by stretching and the retrogradation of the orientation. Thus, there has been employed a process of crystallizing the film prior to stretching as much as possible or subjecting the film after stretched to relaxation annealing to thereby effect removal of the internal strain and recrystallization. As for the process for advancing the crystallization of the film prior to stretching, as far as the aspect of processing is concerned, slow cooling is suitable for cooling the film just after extruded; hence air cooling manner is more advantageous than water cooling manner, and in the case of water cooling manner, cooling has been advantageiously carried out at a relatively high temperature of water. As far as the aspect of raw material is concerned, as for a process for enhancing the crystallization of the film or unstretched yarns, there has been employed a process of adding an organic nucleus-creating agent such as p-tertiary-butyl benzoic acid aluminum salt, dibenzylidene sorbitol, etc. to a conventional polypropylene resin. However, even though the above-mentioned processes are employed and further a relaxation annealing is added thereto, improvement in the heat-shrinkability at high temperatures, higher than 130.degree. C. cannot be observed.